The reason I posted originally was because I had already replaced the existing MCB for a RCD and it doesn't appear to work, it won't trip on the test button but does trip when you switch the shower on. Therefore I just wanted to see if I had done it right. I have no idea why it doesn't work and that is probably the best reason for getting a sparky in, not to mention the dangers etc...
Another question then, what should I be realistically paying someone to come in and sort it?
